CROSS REFERENCE TO BE RELATED APPLICATION
This Application is related to U.S. Ser. Nos. 917,087,filed Sept. 17, 1987 and 928,005, filed Nov. 7, 1986.
1. Field of the Invention
The present invention relates to a display control apparatus employing a bit map method, and more particularly, to a display data transfer control apparatus applicable for a display unit, in which a so-called multi font control, a vertical elongation control of characters, an elongation control of vertical ruled lines, and the like can be performed in a DMA (Direct Memory Access) transfer sequence for transferring character font data from a main storage or pattern ROM (Read-Only Memory) to a video memory (VRAM; Video Random Access Memory). The present invention also relates to a display data transfer control apparatus in which a transfer data processing according to character attributes such as an underline or overline can be performed in addition to the above-mentioned functions in the DMA transfer sequence.
2. Description of the Related Art
A bit map method is known wherein a video memory (VRAM) corresponding to a display screen is provided as a means for displaying a character pattern, a graphic pattern, and the like, on a CRT display, and data on the display screen is temporarily stored in the VRAM and is then read out as a video signal to be displayed on the CRT display. The bit map method requires a 72-byte pattern expansion in order to display one character consisting of, e.g., 24 dots.times.24 dots, unlike a method in which the video signal is directly generated by a character generator in response to the character code. For this reason, a processing speed for expanding a character image in the VRAM is low.
Conventionally, to improve the processing speed, character font data is transferred from a main storage or a pattern ROM to the VRAM by a DMA transfer method. However, an allocation of the transferred character font data as screen data to a predetermined position on the VRAM, a character elongation control or ruled line connection control, and a character attributes processing for a character to be displayed are executed by a dedicated controller respectively.
With the conventional method, however, since the DMA transfer control is performed independently from the character elongation control, ruled line connection control, or a processing control of character attributes such as underline and overline the volume of hardware therefor increases, and it takes a great deal of time to expand a pattern in the VRAM.